This invention relates generally to ultrasound imaging systems and, more particularly, to continuous transmit focusing for ultrasound imaging systems.
Conventional ultrasound imaging systems generally form an image in the following manner. A short acoustic pulse is transmitted into a region of interest from a subset of transducer elements on an array, focused at a particular depth and direction. The acoustic wavefront formed by the superposition of the transmitted pulses propagates along the selected direction and upon backscattering from structures contained within the region of interest, propagates back towards the transducer array (refer to FIG. 1). These echos received from different transducer elements are subsequently amplified and combined using delay, phase, and apodization in such a manner as to provide a dynamic receive focus which changes as a function of time/depth along the transmitted wavefront direction. The combined signal is then log detected and further processed prior to being stored. This process is repeated many times as the transmit and receive directions are changed in such a way as to sweep through the region of interest, i.e., steered, translated, or both. Upon collecting the desired number of line acquisitions, this acoustic data is then scan converted for display to form the resulting ultrasound image. The rate at which these images are formed and displayed is referred to as the frame rate.
For an ultrasound imaging system to produce high quality images, the region of interest must be properly sampled acoustically, both in the range and azimuth (lateral) dimensions in order to prevent aliasing artifacts, which can arise in all sampled systems. In the range dimension, the Nyquist sampling theorem requires that an adequate number of samples in range be acquired based upon the combined round-trip transmit/receive pulse bandwidth. In the azimuth, or lateral, dimension, the Nyquist sampling theorem requires that 1) the region of interest be laterally insonified by a sufficient number of transmit beams and 2) an adequate number of combined round-trip transmit/receive beams laterally sample the region of interest. Stated another way, the Nyquist sampling theorem imposes a transmit acquisition lateral sampling criteria, as well as a round-trip transmit/receive lateral sampling criteria. The Nyquist transmit beam spacing Δxxmt is dependent upon the transmit aperture size Axmt, focusing location rxmt, and the carrier wavelength of the acoustic radiation λ0. It is given byΔxxmt=λ0Fxmt  (1)where the transmit F-number Fxmt=rxmt/Axmt. The Nyquist round-trip beam spacing Δx is dependent upon both transmit and receive F-numbers Fxmt and Frcv respectively. It is given by
                              Δ          ⁢                                          ⁢          x                =                                            λ              0                        ⁢                                                            F                  xmt                                ⁢                                  F                  rcv                                                                                                  F                    xmt                    2                                    +                                      F                    rcv                    2                                                                                =                                    Δ              ⁢                                                          ⁢                              x                xmt                            ⁢                              Δ                rcv                                                                                      Δ                  ⁢                                                                          ⁢                                      x                    xmt                    2                                                  +                                  Δ                  ⁢                                                                          ⁢                                      x                    rcv                    2                                                                                                          (        2        )            where Frcv=rrcv/Arcv for focusing location rrcv and Δxrcv=λ0Frcv. Note that when the receive F-number is much lower than the transmit F-number, the Nyquist round-trip beam spacing Δx is dominated by the receive beam characteristics. This is the direct result of the round-trip transmit/receive beampattern S(ω,x,r) being equal to the multiplication of the individual transmit and receive beampatterns Sxmt(ω,x,r,rxmt) and Srcv(ω,x,r) respectively, at a particular frequency f and range r, and is given by
                                          s            ⁡                          (                              t                ,                x                ,                r                            )                                =                                    1                              2                ⁢                π                                      ⁢                                          ∫                                                                                                                                ⁢                                                S                  ⁡                                      (                                          ω                      ,                      x                      ,                      r                                        )                                                  ⁢                                  ⅇ                                      jω                    ⁢                                                                                  ⁢                    t                                                  ⁢                                  ⅆ                  ω                                                                    ⁢                                  ⁢                              S            ⁡                          (                              ω                ,                x                ,                r                            )                                =                                                    S                xmt                            ⁡                              (                                  ω                  ,                  x                  ,                  r                  ,                                      r                    xmt                                                  )                                      ⁢                                          S                rcv                            ⁡                              (                                  ω                  ,                  x                  ,                  r                                )                                                    ⁢                                  ⁢                                            s              xmt                        ⁡                          (                              t                ,                x                ,                r                ,                                  r                  xmt                                            )                                =                                    1                              2                ⁢                π                                      ⁢                                          ∫                                                                                                                                ⁢                                                                    S                    xmt                                    ⁡                                      (                                          ω                      ,                      x                      ,                      r                      ,                                              r                        xmt                                                              )                                                  ⁢                                  ⅇ                                      jω                    ⁢                                                                                  ⁢                    t                                                  ⁢                                  ⅆ                  ω                                                                    ⁢                                  ⁢                                            s              rcv                        ⁡                          (                              t                ,                x                ,                r                            )                                =                                    1                              2                ⁢                π                                      ⁢                                          ∫                                                                                                                                ⁢                                                                    S                    rcv                                    ⁡                                      (                                          ω                      ,                      x                      ,                      r                                        )                                                  ⁢                                  ⅇ                                      j                    ⁢                                                                                  ⁢                    ω                    ⁢                                                                                  ⁢                    t                                                  ⁢                                  ⅆ                  ω                                                                                        (        3        )            where x is the lateral spatial coordinate, t is the time coordinate, ω=2πf, and the transmit, receive, and round-trip point spread functions (PSF) are the inverse Fourier transforms of their respective beampatterns. If the receive beam is much narrower than the transmit beam, then the receive beam dominates the round-trip beampattern. FIG. 2 depicts the situation where the receive beam F-number is substantially lower than the transmit F-number and as such, the receive beam, which is typically dynamically focused, i.e., the aperture size and focal point are increased as a function of time/range so as to maintain constant lateral resolution Δxrcv, dominates the round-trip beam pattern. The transmit beam is generally focused at a user specified depth. Note the reduction in the round-trip sidelobe clutter energy outside of the round-trip beampattern's mainlobe at the transmit focus location, as compared to away from the transmit focus location. This is due to the multiplicative influence of the transmit beam on the receive beam. The transmit beam can have a more significant influence on the receive beam if lower transmit F-numbers are employed. However, since the transmit beam is not dynamically focused for all depths, the round-trip beampattern will display much better lateral resolution at the transmit focus location compared to away from the transmit focus location. For example, if Fxmt Frcv, Δx=Δxrcv/√2 at the transmit focus location, and Δx=Δxrcv away from the transmit focus location—a 41% degradation in lateral resolution, as well as increased sidelobe clutter. This leads to lateral image non-uniformity, which is undesirable for high quality ultrasound imaging.
Conventional ultrasound imaging systems which form a single receive beam for each transmit beam as depicted in FIG. 2, necessarily couple the Nyquist transmit beam spacing Δxxmt to the Nyquist round-trip beam spacing Δx. Instead of having to fire a minimum of L/Δxxmt transmit beams to adequately insonify the region of interest, the single beam conventional system fires L/Δx transmit beams to cover the region of interest, where L is the lateral extent of the region of interest to be imaged. Using (1) and (2), the potential acoustic acquisition rate is reduced by the factor
                    η        =                                            1              +                                                F                  xmt                  2                                                  F                  rcv                  2                                                              =                                    1              +                                                Δ                  ⁢                                                                          ⁢                                      x                    xmt                    2                                                                    Δ                  ⁢                                                                          ⁢                                      x                    rcv                    2                                                                                                          (        4        )            Using a typical single focus example of Fxmt=2.5 and Frcv=1.0, the reduction in the potential acoustic acquisition rate using (4) is ˜2.7. Almost three times as many transmit beams are being fired than required by Nyquist sampling requirements.
If the transmit and receive F-numbers are made equal, i.e., Fxmt=Frcv=1.0, then the reduction in potential frame rate using (4) is ˜1.4, which is not as dramatic as when the receive F-number is much lower than the transmit F-number; however, an additional problem is introduced, namely the limited depth of field of the transmit beam. While the receive beam is dynamically focused, the transmit beam is focused only at a single depth. The range r over which the transmit beam can be considered “in focus” is given by the depth of field expression
                                                        R              DOF              xmt                        ∼                          βλ              ⁢                                                          ⁢                              F                xmt                2                                              ,                      4            ≤            β            ≤            8                          ⁢                                  ⁢                                            r              xmt                        -                                          R                DOF                xmt                            2                                ≤          r          ≤                                    r              xmt                        +                                          R                DOF                xmt                            2                                                          (        5        )            where the choice of β depends upon what phase error is assumed at the end elements of the transmit aperture in the depth of field (DOF) derivation. Note the DOF dependence on the square of the transmit F-number. As the transmit F-number decreases, the transmit beam's lateral resolution increases linearly as given by (1); however, the range over which the transmit beam will be in focus, and have influence on the round-trip lateral resolution given by (2), decreases quadratically. This introduces image lateral non-uniformity and higher clutter away from the transmit focus and is undesirable in high quality ultrasound images. The conventional approach to mitigate this behavior is to transmit multiple times along each transmit beam, changing the transmit focus location rxmt on each firing and performing a conventional receive beam formation on each. The resulting multiple round-trip signals are then typically combined following the detection process, with the DOF image region around each transmit focus rxmt being retained, with the rest discarded. Thus, each detected line is the composite of multiple lines, each having a different transmit focus. The number of transmit firings along the same transmit beam direction that are required is dependent upon the desired transmit F-number to be supported, and the display range of the region of interest, with the number of transmit firings going up as the square of the transmit F-number reduction. This produces a significant loss in acoustic acquisition rate due to the multiple transmit firings along the same line.